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Eagle1
30th Jul 2001, 00:04
Hi all,

Just discovered this great tech-log, with lots of surprising questions and even more unimaginable answers. Very interesting, and it wets my appetite to pose a question which has been on my mind for some time:

Am I right in thinking that when an airplane has a lower weight, the manouvering speed (Va) is lower as well? I'd say it would be the opposite: with a lower weight I should be able to throw the airplane around with a higher speed before full, sudden deflection of the rudders becomes dangerous. Or does the lower weight make an airplane more mavouverable, so Va is reached quicker?
What's the clue?
If there is anyone out there who knows an answer, I'd be happy to hear. In the meantime I keep reading the other questions, knowing that I'm not the only one who has difficulty falling asleep with thougts like these on my mind.
Greetings!

30th Jul 2001, 00:34
Your reasoning is quite correct, and you've almost answered your own question!

Va is nothing to do with the rudders, it is the speed at which a pull to the limit load factor (2.5g for public transport a/c) will result in a stall. A speed less than this, and it you will stall before you reach the limit load factor, above this speed it is possible to overstress the aircraft.

The important thing is that Va relates to the limit load FACTOR, and not the actual load on the structure. eg) 2.5g for public transport. At a lower weight, the stall speed is lower. Therefore, you can pull more 'g' before you stall. This means that Va must be lower (the speed at which a pull of 2.5g just results in a stall).

The flight envelope Va is calculated for the maximum design weight. It is interesting to note that the at a lower weight, the load on the structure will be less at the limit load factor. eg) the weight of the aircraft times the limit load factor is less. Therefore, the Va to give the same limit load FACTOR decreases with weight, but the Va to give the same limit LOAD (in Kg) is independent of weight.

Complicated I know, but hope this helps. :D

john_tullamarine
30th Jul 2001, 02:15
Actually .. a little more complex than that ... Va can be limited by rudders and ailerons .. and do remember, for instance, that the bits bolted on here and there ... aren't looking at the gross weight when it comes to attachment loadings ....and I wouldn't go too far above the manufacturer's figures .. things like wing torsion come to mind ...

Eagle1
30th Jul 2001, 19:06
Ahaaa..... Indeed not easy, but I'm trying to follow. With lower weight stallspeed is also lower, so speed where airplane stalls with full deflections is lower as well... makes sense to someone who skipped maths already years ago. But, next question, if you still have a few seconds: what is the difference between load in actual kilograms and loadfactor?

Wino
30th Jul 2001, 21:33
While it is true that the wing could support the same load which would result ina higher G loading.

The battery mount for example is still limited to 2.5 Gs and is carrying the same load. Same thing for the engine mount.

Who says the wing has to be the first thing to fail?

Cheers
Wino

john_tullamarine
30th Jul 2001, 22:46
Eagle1,

The normally quoted Vs data (a la POH/AFM numbers) are for 1g slow pitch rate approaches to CLmax alpha. If we look at Vs variation with GW, it generally follows something along the lines of square root GW. If we now look at an acceleration other than 1g, then we ought to be interested in the product of load factor and GW, ie nW. I guess that you could think of this quantity as being some sort of 'effective' gross weight.

Va is the most critical of a number of design cases to be considered but, in general, the limiting case usually ends up being that where, with

(a) limit load factor (that acceleration for which the GW structure is designed and tested to demonstrate that it doesn't bend too much), and

(b) MTOW,

CLmax arrives. The significance is that, up to Va, the hamfisted pilot has a degree of protection against his absence of TLC - remember how your initial instructor used to exhort you to treat the aircraft like an elegant lady ? ... good training in life skills for a young chap ...

If you now reduce the GW, then you could increase the load factor to keep the stall occurring at the same Va speed. However, this would then impose, potentially and probably, excess attachment loads, in particular, on the various bits and pieces bolted on here and there throughout the airframe.

As to your second question, the loads are normally measured in newtons, kilogrammes being a mass unit, although we are all comfortable with thinking in terms of kilogrammes as being a force unit. The load factor is the vertical acceleration factor compared to the 1g case as illustrated above .... It might help in your coming to grips with this idea if you consider the case of a lump of something on the end of a bit of string. With the string held stationary, the lump hangs down and you feel its 1g weight. Now, if you swing it around your head, the tension load in the string increases above the previous 1g load. You can think of the ratio of the two loads as being something analogous to load factor.

Did I succeed in making anything easier to understand ? .. or did I just muddy the waters further ?

Eagle1
31st Jul 2001, 22:21
Wow... Well, the situation is somewhere between muddy waters and swampy ground, I'd say. I get a general picture - you guessed already I'm not an ATPL of some kind, let alone an engineer; 'just' PPL applies to me.
Still interested in these things, so I'll keep track of what's being said in this forum. Thanks very much for your time and help.
Greetz, Matthijs

UP2ZSKY
1st Aug 2001, 07:38
On the issue, what is more important to me, is how Va changes with altitude, not weight. My aircraft varies from Va 196 @ sl to something like 220 FL410.
It is not published in the flight manual but I found it in some obscure certification documentation.

john_tullamarine
1st Aug 2001, 18:00
UP2ZSKY,

Never looked at that aspect and can't recall seeing such data. However, a possible explanation is that CLmax reduces at higher levels due to air properties variations which we ignore for lower levels. As a result the stall speed increases, as would the speed at which the limit load factor can be achieved.

Genghis the Engineer
4th Aug 2001, 11:34
Not rather an issue of PEC variation with sigma perhaps?

G